Almost all aviation turbine fuels yet fuels) are currently made from fossil sources, with most of it being refined from crude petroleum and a small amount derived from other sources like coal or natural gas. Jet fuels from refined petroleum and in particular kerosene-type jet fuels are currently preferred because they offer the best combination in terms of energy content, performance, availability, ease of handling and price. The past increases in the price of petroleum, concerns about its future availability and security of supply as well as concerns with regard to the emission of greenhouse gases and emission of pollutants have prompted governments and industry to look for alternatives.
For economic as well as safety reasons, alternative aviation turbine feels have to be suited for use with conventional turbine engines, i.e. without requiring any modification of the engines, and have to show the same essential fuel performance properties than conventional jet fuel. In other words, alternative aviation turbine fuels have to comply with the major specifications for commercial jet fuel as issued by ASTM (American Society for Testing and Materials), MOD (United Kingdom Ministry of Defence), or GOST (Gosudarstwenny Standart).
Irrespective of whether conventional or alternative aviation turbine fuels are concerned, the primary function of any jet fuel is to provide a source of chemical energy for propelling a jet aircraft. The key fuel performance properties are therefore energy content and combustion quality. Other essential fuel properties are homogeneity, stability, lubricity, fluidity, cleanliness, and safety properties.
The energy content of a fuel determines how far an aircraft can fly and is expressed either gravimetrically as energy per unit mass of fuel or volumetrically as energy per unit volume of fuel. The combustion quality concerns the radiant heat transfer in turbine engines and is correlated with the flame temperature, the formation of carbonaceous particles in the process of combustion and the formation of smoke and soot. Stability requires that the fuel properties remain unchanged over time and when exposed to high temperatures in the engine. One of the stability requirements is homogeneity, which means that components concerned are miscible with each other and there is no phase separation in the applicable temperature range. Since jet engines rely on the fuel to lubricate some moving parts in fuel pumps and flow control units, aviation turbine fuels have to feature some lubricity. Fluidity concerns a fuel's ability to be freely supplied from the fuel tanks to the turbine engines of an aircraft, since otherwise an aircraft engine would not able to function. Fluidity concerns the low temperature stability of a fuel usually characterised by its freezing or clouding point below which one of the fuel components solidifies, its viscosity, volatility, and its non-corrosivity, that is its ability not to affect any materials present in the fuel and combustion systems. Fuel cleanliness means the absence of particulates like rust, dirt, and microorganisms, and free water or water-fuel emulsions in the fuel that can plug fuel filters and increase fuel pump wear. Safety properties concern the handling of the fuel and in particular its ignitability characterised by the flash point temperature and its ability to prevent formation of static charges.
The carbon dioxide impact on the environment due to the combustion of fossil fuels in an aircraft is primarily given by the amount of carbon in the fuel consumed in the combustion process and the carbon dioxide produced upon refining and transportation of the raw materials and distribution of the final product. Efforts have therefore been made to reduce the carbon dioxide impact to below the amount of carbon dioxide produced upon manufacture and combustion of jet fuel. One promising attempt is the manufacture of jet fuel as a whole or in part from renewable resources, the stock of which may be regenerated over a short period on the human scale, with the materials of the renewable resources corresponding to organic materials whose carbons come from non-fossil resources (see ASTM D 6866). The carbon dioxide impact on the environment can particularly be reduced when using jet fuel or jet fuel components derived from biomass, since its carbon content has been obtained by capturing atmospheric carbon dioxide through photosynthesis.
A respective manufacture of renewable biofuels is for instance disclosed in the International Publication WO 2009/079213, where saturated C8-C24 aliphatic hydrocarbons and aromatics are produced from renewable alcohols (with low levels of olefins) derived from biomass. The biofuel can be used as on-specification fuel either alone or blended with petroleum-derived fuels (e.g. jet fuels).
The term biofuel is understood as meaning a renewable transportation fuel resulting from biomass conversion. Renewable fuels are characterised by comprising carbon of renewable origins, that is to say identifiable by the 14C content. Carbon taken from living organisms and in particular from plant matter used to manufacture renewable fuel is a mixture of three isotopes, 12C, 13C, and 14C being kept constant at 1.2·10−12 by the continuous exchange of the carbon with the environment. Although 14C is radioactively unstable with its concentration therefore decreasing over time, with a half-life of 5,730 years, so that the C14 content is considered to be constant from the extraction of the plant matter up to the manufacture of the renewable fuels and even up to the end of their use. A fuel can be designed as renewable fuel or biofuel when the 14C/12C ratio is strictly greater than zero and smaller or equal to 1.2·10−12.
It has been found that replacing portions of the hydrocarbons in motor fuels, such as diesel oil and gas oil, with alcohol compounds provides a cleaner exhaust emission and does not adversely affect engine performance. The widely available and inexpensive alcohols, methanol and ethanol, are however immiscible with diesel and gas oil fuels resulting in an initial unstable homogeneity of the motor fuel. The European Patent Specification EP 1 218 472 B2 therefore suggests to use a blend of oxygen-containing compounds comprising at least four oxygen-containing functional groups, wherein those groups are contributed to by four different oxygen-containing compounds, each of which contains at least one of said groups, by employing at least four types of organic compounds differing in functional groups containing bound oxygen. The blend can be used for operating diesel, gas-turbine, and turbojet engines either alone or combined with a hydrocarbon component.
Another approach is disclosed in U.S. Pat. No. 6,896,708, where particularly selected so-called non-linear long-chain saturated alcohols (NLA) are used in fuel compositions for internal combustion engines.
U.S. Pat. No. 8,277,522 suggests a mixture of mixed alcohol formulations that can contain combinations of two or more or three or more alcohols, or a blend of C1-C5 alcohols, C1-C8 alcohols, or higher C1-C10 alcohols. The mixed alcohol formulations can be used as fuel additive in petroleum and other fuels like e.g. jet fuel or as a neat fuel in and of itself. The primary benefits of the mixed alcohols are said to be increased combustion efficiencies, improved fuel economies, reduced emission profiles and low production costs. Since the presence of oxygen renders the energy content of the lower alcohols methanol (C1) and ethanol (C2) relatively low, the higher alcohols are used to boost the energy content.
In the light of the above it is therefore desirable to provide an aviation feel composition requiring no modification of currently used turbine engines and having, when compared to currently approved aviation fuels, at least one of the following advantages: lower carbon dioxide impact on the environment, lower emission of harmful exhaust gases, and improved characteristics.